Control of Parathyroid Secretion by Calcium Ion Concentration
Even the slightest decrease in calcium ion concentra-tion in the extracellular fluid causes the parathyroid glands to increase their rate of secretion within minutes; if the decreased calcium concentration per-sists, the glands will hypertrophy, sometimes fivefold or more. For instance, the parathyroid glands become greatly enlarged in rickets, in which the level of calcium is usually depressed only a small amount; also, they become greatly enlarged in pregnancy, even though the decrease in calcium ion concentration in the mother’s extracellular fluid is hardly measurable; and they are greatly enlarged during lactation because calcium is used for milk formation.
Conversely, conditions that increase the calcium ion concentration above normal cause decreased activity and reduced size of the parathyroid glands. Such con-ditions include (1) excess quantities of calcium in the diet, (2) increased vitamin D in the diet, and (3) bone absorption caused by factors other than PTH (for example, bone absorption caused by disuse of the bones).
Figure 79–11 shows the approximate relation between plasma calcium concentration and plasma PTH concentration. The solid red curve shows the acute effect when the calcium concentration is changed over a period of a few hours. This shows that even small decreases in calcium concentration from the normal value can double or triple the plasma PTH. The approximate chronic effect that one finds when the calcium ion concentration changes over a period of many weeks, thus allowing time for the glands to hypertrophy greatly, is shown by the dashed red line; this demonstrates that a decrease of only a fraction of a milligram per deciliter in plasma calcium concentra-tion can double PTH secretion. This is the basis of the body’s extremely potent feedback system for long-term control of plasma calcium ion concentration.